The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile employing electrophotography and, more specifically, an image forming apparatus provided with an intermediate transfer member.
In the above-described image forming apparatus, a toner image primarily transferred from a latent image carrier such as a photosensitive drum to an intermediate transfer member such as an intermediate transfer belt is secondarily transferred to a recording medium such as paper. After then, toner remaining on the intermediate transfer member is removed by a cleaner such as a cleaning blade which comes into contact with the surface of the intermediate transfer member and scraping the residual toner from the intermediate transfer member.
FIG. 1A shows a state in which a cleaning blade 14 comes into contact with an intermediate transfer member 12 suspended by a driving roller 10 and a follower roller 11. As shown in FIG. 1C, toner T is accumulated on the extremity of the cleaning blade 14 by the amount corresponding to the thickness of the blade. From this state, as shown in FIG. 1B, when the cleaning blade 14 is separated from the intermediate transfer member 12, a toner line (separation line) 24 is generated. As shown in FIG. 1D, the width L of the toner line 24 is substantially equal to the thickness W of the cleaning blade 14.
Consequently, there arises a problem that the toner line 24 overlaps a toner image which is to be primarily transferred to the intermediate transfer member 12 in the subsequent image forming process. Japanese Patent Publication Nos. 2000-231276A and 2002-82533A teach that the separation timing of the cleaner is determined with reference to the position of the toner image to be primarily transferred to the intermediate transfer member 12, so as to prevent the toner line generated by the cleaner from overlapping the image area.
However, as shown in FIG. 2A, the toner line 24 formed in the above-described non-image area may attached to a secondary transfer roller 15 when the secondary transfer roller 15 comes into contact with the intermediate transfer member 12 immediately before the secondary transfer operation. Then, when the secondary transfer operation to a recording medium S is completed, and the secondary transfer roller 15 is separated from the intermediate transfer member 12 as shown in FIG. 2B, the toner line 24 is again attached on the intermediate transfer member 12.
FIG. 3A shows a state that a position C1 at which the toner line 24 is attached proceeds toward a primary transfer position T1 which is defined by an image carrier (photosensitive drum) 3 and a primary transfer roller 13. FIG. 3B shows a state that the toner line 24 reaches the primary transfer position T1, and an electrostatic latent image LI is formed by an exposure operation (i.e., light beam irradiation as indicated by an arrow). FIG. 3C shows a state that the latent image LI is developed by a developing roller 6a as a visible toner image TI.
The rotation velocity of the image carrier 3 changes when the image writing (exposure operation) is performed while the toner line 24 is at the primary transfer position T1 and the width L of the toner line 24 is no less than the nip width N of the primary transfer position as shown in FIG. 3D. Consequently, unevenness of density or color shifting due to the rotation velocity fluctuations of the image carrier 3, that is, so-called banding stain occurs. It results from the fact that when there exists the toner line 24 between the intermediate transfer member 12 and the image carrier 3, a friction force between them is lowered, so that the image carrier 3 slips and results in the rotation velocity fluctuations.
FIG. 4 shows experimental data obtained by measuring rotation velocity fluctuations of the image carrier 3. In this experiment, the circumferential velocity of the intermediate transfer member is set to a value faster than that of the circumferential velocity of the image carrier by 0.7%, and the toner images are transferred to the intermediate transfer member in the order of Bk (black), C (cyan), M(magenta), and Y(yellow).
In FIG. 4, the vertical axis represents rotation velocity fluctuations, the lateral axis represents time, and the rotation velocity fluctuation is obtained by subtracting the rotation velocity of the image carrier from the circulation velocity of the intermediate transfer member, and the result is then divided by the rotation velocity of the image carrier. Finally, the result is multiplied by 100 to obtain a percentage value. Accordingly, the “plus” value implies that the image carrier is slower than the intermediate transfer member, and the “minus” value implies that the image carrier is faster than the intermediate transfer member. The significant rotation velocity fluctuations appeared in the initial period are turbulence of an encoder signal occurred at the position corresponding to a seam of the intermediate transfer member (belt). The turbulences may be ignored because they are not actually the rotation velocity fluctuations. When the position of the toner line 24 on the intermediate transfer member 12 reaches the primary transfer position T1, distinctive rotation velocity fluctuations can be observed as shown by arrows A.
FIG. 5A shows experimental data in a case where the toner line formed on the intermediate transfer member is not removed. FIG. 5B shows experimental data in a case where the toner line formed on the intermediate transfer member is removed before it reaches the primary transfer position. From both results, it is apparent that the rotation velocity fluctuation pointed by an arrow is derived from the existence of the toner line.
The banding stain problem described above also occurs when the toner mark is attached to the area on the image carrier 3 corresponding to the non-image area on the intermediate transfer member 12. As shown in FIG. 6A, when the developing roller 6a comes into contact with a position C2 on the image carrier 3, a toner line (contact line) 24 is attached thereto due to the impact of the developing roller 6a. As shown in FIG. 6B, the toner line 24 then reaches the primary transfer position T1. As shown in FIG. 6C, the toner line 24 is partially transferred to the non-image area on the intermediate transfer member 12 and passed through the primary transfer position T1, while the electrostatic latent image LI is developed by the developing roller 6a. 
Similarly, the rotation velocity of the image carrier 3 changes when the image writing (exposure operation) is performed while the toner line 24 is at the primary transfer position T1. Consequently, unevenness of density or color shifting due to the rotation velocity fluctuations of the image carrier 3, that is, so-called banding stain occurs. It results from the fact that when there exists the toner line (contact line) 24 between the intermediate transfer member 12 and the image carrier 3, a friction force between them is lowered, so that the image carrier 3 slips and results in the rotation velocity fluctuations.
In the experimental data shown in FIG. 7, the pressure fluctuations due to the contact line 24 can be observed at positions pointed by arrows B.
In such an image forming apparatus that an AC-superimposed bias is applied to a developing roller to develop an electrostatic latent image as a visible toner image, toner may locally attach to an image carrier by splashing or fogging of toner since the high voltage level of the bias cannot be stabilized at the initial stage of the application of the developing bias. In view of the above, Japanese Patent Publication No. 3-64073B teaches that an AC-superimposed bias is applied before the latent image on the image carrier reaches the developing position (that is, applied at a position corresponding to a non-image area) for stabilizing the bias before development, so that splashing or fogging of toner is prevented.
However, as shown in FIGS. 8A and 8B, when the AC-superimposed bias is applied to the position corresponding to the non-image area, toner in an area Y splashes on the image carrier 3 at the initial stage of bias application. Since the developing roller 6a and the image carrier 3 rotate in the direction indicated by arrows and a gap between the image carrier 3 and the developing roller 6a is increased, thereby weakening an electric field, the splashed toner cannot return to the developing roller 6a. Also, since the power source may become unstable at the initial stage of bias application, so-called overshoot may occur, and hence toner may splash to the image carrier 3. The toner line (bias application line) 24 is thus formed in the non-image area of the image carrier 3.
As shown in FIG. 9A, when the developing bias generated from a bias power source 25 is applied at a position C3 on the image carrier 3 via the developing roller 6a, the toner line 24 is attached due to the impact of the image carrier 3. As shown in FIG. 9B, the toner line 24 then reaches the primary transfer position T1. As shown in FIG. 9C, the toner line 24 is partially transferred to the non-image area on the intermediate transfer member 12 and passed through the primary transfer position T1, while the electrostatic latent image LI is developed by the developing roller 6a. 
The rotation velocity of the image carrier 3 changes when the image writing (exposure operation) is performed while the toner line 24 on the position C3 is at the primary transfer position T1. Consequently, unevenness of density or color shifting due to the rotation velocity fluctuations of the image carrier 3, that is, so-called banding stain occurs. It results from the fact that when there exists the toner line (bias application line) 24 between the intermediate transfer member 12 and the image carrier 3, a friction force between them is lowered, so that the image carrier 3 slips and results in the rotation velocity fluctuations.
In the experimental data shown in FIG. 10, the pressure fluctuations due to the bias application line 24 can be observed at positions pointed by arrows C.